Compression Height

427John wrote:While I can't help you with advice I'm curious on your combination.Why the .050 destroke,is there a 470 limit on your class and thats what you had to do to get there with the bore you had?Also about your compression height question are you shooting for the shortest lightest piston possible with the longest rod?

Dave De wrote:Running a compression height at 1.35" is about as low as you can go and not use support rails with a .990 pin diameter. This is going to require a 7.05 rod with a 10.300 deck height. If you are going custom pistons and a 7.00" rod then a 1.40" CH will go zero in the hole.
That is one long rod with the piston spending a long time at both ends.

EverySparePenny wrote:470" truck puller deal. 4.440 bore, 3.8 stroke, 10.3XX deck height, all OE iron.  800+hp, 16ish:1 compression, 8000-9000 rpm operating range for approximately 30 second passes.

What is the shortest recommended compression height?  Custom dome piston obviously.

Thank you

Dave De wrote:Running a compression height at 1.35" is about as low as you can go and not use support rails with a .990 pin diameter. This is going to require a 7.05 rod with a 10.300 deck height. If you are going custom pistons and a 7.00" rod then a 1.40" CH will go zero in the hole.
That is one long rod with the piston spending a long time at both ends.

Mark O'Neal wrote:1.250 is about as short as I care to go, with rail supports (and there is no reason to avoid oil rail supports.) But even the 7.150 that you would need to run 1.250 is a total waste of effort. 7.100 rods are still available, from Molnar and others....if you really feel the need. The C/H would be about 1.290.

7.700 will work, but the pin will have to be in the deck of the piston with the top of the rod sticking out....that and you won't be able to run rings, no room for those. Would clear the crank very well, as an upside. That and pin engagement would be right at 100%.

As for where everything hangs out at top and bottom dead center..that doesn't matter.

Paul Kane wrote:A 7.1 steel rod will work fine and make piston selection easy.

If you want aluminum rods shorter than what you have then I have a set of Childs & Albert Series 200 7.25" rods. They will require a piston with a 1.15" pin location, which can be done....but the steel 7.1" rod & shelf piston (with a single change) is the easier way to go.

gt350hr wrote:

Mark O'Neal wrote:1.250 is about as short as I care to go, with rail supports (and there is no reason to avoid oil rail supports.) But even the 7.150 that you would need to run 1.250 is a total waste of effort. 7.100 rods are still available, from Molnar and others....if you really feel the need. The C/H would be about 1.290.

7.700 will work, but the pin will have to be in the deck of the piston with the top of the rod sticking out....that and you won't be able to run rings, no room for those. Would clear the crank very well, as an upside. That and pin engagement would be right at 100%.

As for where everything hangs out at top and bottom dead center..that doesn't matter.

   All this from the guy who's company started the whole pin in the ring is an issue deal! My how times have changed.
7.100 is the practical limit for rod length. One thing to consider is the torque band rpm rises and becomes more narrow with the long rod than with the  6.7-6.8 combination which could be a problem as the weight comes forward on the sled. Matching piston speed to port flow ( affected by rod length ) is FAR more important than some minimal amount of time extra "burn time". Besides energy isn't being when the piston isn't moving.

We also wanted to point out some of the common myths and misconceptions about high-performance motors. For example, I’ve seen dozens of magazine articles on supposedly “magic” connecting rod ratios. If you believe these stories, you would think that the ratio of the connecting rod length to the crankshaft stroke is vitally important to performance. Well, in my view, the most important thing about a connecting rod is whether or not the bolts are torqued!

If I had to make a list of the ten most important specifications in a racing engine, connecting rod length would rank about fiftieth. Back in the days when Buddy Morrison and I built dozens of small-block Modified motors, we earnestly believed that an engine needed a 1.9:1 rod/stroke ratio. Today every Pro Stock team uses blocks with super-short deck heights, and we couldn’t care less about the rod ratio. A short deck height improves the alignment between the intake manifold runners and the cylinder head intake ports, and helps to stabilize the valvetrain. These are much more important considerations than the rod-to-stroke ratio. There’s no magic – a rod’s function is to connect the piston to the crankshaft. Period.

gt350hr wrote:Typical drag race mentality. I will NEVER know as much as David Reher , NEVER. Unfortunately every engine isn't an NHRA Pro Stock engine There are at least a hundred dirt late model , sprint car , and road race engine builders that will disagree with David's opinion. An engine that runs from 8-10,500 rpm for 6.5 seconds and makes four closely split gear changes wouldn't be very sensitive to rod length. The statement is taken out of context similar to often quoted one from the late Smokey Yunick saying you always need to run the longest possible rod.